Breathe. Was that your last one? If not get ready for the next one.
That one's over. Can't do anything about it. Might have done good enough. Just wait for your grade. Remember, you don't have to outrun the bear. You aren't measured against the material. You're measured against your classmates. If you ran out of time, some of them probably did, too.
In the mean time, put it out of your mind and get ready for what's next.